In known automotive internal combustion engines, air-fuel mixture has not been ignited and burned readily, especially during idling and light load operation when throttle opening is decreased so that less air-fuel mixture is introduced into the combustion chamber at lower speed.
Usually, this poor ignition and combustion problem has been solved by feeding rich mixtures, i.e., mixture having low air-fuel ratios, that have enhanced combustibilities. But this solution is accompanied by increased fuel cost and increased emission of toxic, unburned hydrocarbons, carbon monoxide and so on.
To control noxious nitrogen oxide emissions, it has also been proposed to burn mixtures sufficiently leaner than the stoichiometric mixture, and to burn again part of exhaust gas by taking it out of the exhaust system and mixing with air-fuel mixture. But the low ignitability and combustibility inherent to both methods have likewise impaired driveability and fuel economies.